Re-installing doesn't erase your array. You are supposed to disconnect all other drives except system when installing anyway.

This means
1) disconnection of every hdd (system is installed at a usb-stick SLC nand flash)
2) installation of OMV
3) re-connection of the six hdd with the raid-6-array
4) raid-6-array is shown at 'filesystems' at settings so it can be mounted and accessed

I'm a little bit worried as I read that there are several commands needed such as 'mdadm --assemble --scan' after installation? This is said to not have the raid-6 accessible...
Do you know of a howto to guide me through this process?

Has this issue with Plex been solved yet? The same thing is happening to me with the latest version of Erasmus. When I install the plexmediaserver plugin it never completes. I have to close the web page and re-enter the web gui. when I do that I get:

I tried the previous statements by @ryecoaaron and it still having the same issue. I did a fresh install and i can't get plex to install. Causes the same errors as above.

Not sure what previous statements you are referring to or what system you are installing on. It does seem that people who are using libraries from previous installs have a lot of issues (probably permissions or ownership).

After having tried plexmediaserver in docker I wanted to return to standard plugin installation in OMV 3.

There is an issue already during the re-installation process resulting in a.m. error. As it seems, only re-installation is affected
(i had removed plexmediaserver before my docker try via UI). Perhaps there is an issue in the de-installation that prevents a proper
re-installation?

Plex installed fine the first time but the web interface wasn't working so I uninstalled and reinstalled, and ever since then it has 'failed' the reinstall. It installs ending in an error, but then the services page will not load in OMV until plex is uninstalled. Activating and unactivating the plex service doesn't do anything. If I uninstall and reinstall I get the error every time.